Field of the Invention
The invention relates to a method for operating a direct-injection internal combustion engine under starting conditions. The internal combustion engine is capable of being started selectively with homogeneous or stratified operating conditions and predetermined engine control parameters for the homogeneous and the stratified operation are calculated for this purpose.
Internal combustion engines with direct injection have great potential for reducing fuel consumption, while at the same time ensuring relatively low pollutant emission. In contrast to manifold injection, in direct injection fuel under high pressure is injected directly into the combustion chamber.
Direct injection, by giving a free choice of injection timing, makes it possible for the internal combustion engine to run in various operating modes. In the case of very early injection during the intake stroke, substantial charge homogenization is achieved, and, consequently, homogeneous operation which is used for maximum air utilization under full load, sometimes also during cold starting and warming up, and, in particular, for homogeneous part-load operation. In the case of late injection timing during the compression stroke, more or less pronounced charge stratification is obtained, and, consequently, stratified operation which is employed largely in extremely lean part-load operation. The ignition timings may also be selected differently (see, in this respect, AVL Tagung "Motor und Umwelt" [AVL Conference "Engine and Environment"] 1997, "Otto-Direkteinspritzung--ein ganzheitlicher Systemansatz" ["Direct Gasoline Injection--An Overall System Approach"], pages 255-271).
In direct-injection internal combustion engines it is known, furthermore, for the starting process to take place in the homogeneous operating mode. In this case, the fuel injection quantity and the angle of commencement of injection are predetermined, and injection commences immediately after synchronization is concluded.
Since, immediately after the starting process has commenced, because of the mechanical drive of the fuel high-pressure pump the pressure in the high-pressure accumulator (rail pressure) is determined essentially by the electrically driven feed pump and therefore has relatively low values, the injected fuel is inadequately prepared (large fuel droplets). The duration of injection, and consequently also the end of injection, changes, depending on the rail pressure. The result of this, under some circumstances, is that there is no ignitable mixture present at the sparkplug and there is a very long timespan until the starting process has ended. Such a starting process is also termed low-pressure starting.